In typical applications, so-called "add-on," as opposed to "in-hull," transducers are mounted just aft and below the transom of a boat hull for measuring the characteristics of the water or of the boat performance, i.e., characteristics such as water depth, water temperature, speed through the water, and the size and location of marine life, among others. Although the discussion below will center on transducers which provide a display or other read-out of what is taking place under the boat, the invention is not limited to such applications.
Sonar devices, variously referred to as "fish finders" or "depth sounders," have been available for some time which are used to tell a user what is below the boat. Transducers used to transmit acoustical energy (sonar) into, and receive this energy from, the water can either be an integral part of the boat hull (the "through-the-hull" or "in-hull" design) or mounted by means of appropriate brackets to the lower part of the boat transom (the "transom-mount" design). Such "transom-mount" designs are most commonly used by sport fisherman and much of the discussion below is concerned with such designs.
Typically, transducers designed for transom-mounting have plastic or bronze housings simply designed to protect the piezo-electric crystal or crystals of the transducers from water and physical damage. The transducer crystal(s) and the bonding envelope therefor are usually bonded inside the outer housing with a plastic compound to form a continuous "solid" medium through which the acoustical energy waves from the crystal can be transmitted. Typically, a relatively soft material, e.g., cork, is used to envelope the crystal sides and top such that a large acoustical impedance mismatch occurs, causing sound waves to be reflected back into the crystal, while at the same time communication (or reception) of the generated (or reflected) acoustical wave takes place through the transducer structure.
The outside shape of many commercial transducers is typically flat-bottomed with a knife-edge or a like sharp taper being provided at the leading edge for "flow control" over the transducer surface. The transducer is most often mounted below the hull of the boat with a slightly positive angle of attack relative to the oncoming water.
Briefly considering the history of such sonar transducers, one of the earliest sonar transducer devices made by the assignee of this application was a portable low-power unit that a fisherman could carry with him and use on most boats. The output signal presented to the fisherman was a rotating flashing light that represented the presence and/or depth of the fish and bottom. Hence, most models employing this sort of output are generically referred to as "flasher" units.
In those days, the popular way of fishing was to rent a rowboat, possibly with a small outboard motor. Many fishermen had their own small motors but most did not have their own boats and thus a portable sonar unit that could be carried from boat to boat was extremely popular at that time. The sonar device operated off of flashlight batteries and was of very low transmit power. The transducer was a cylindrical unit, flat on the bottom (the bottom end of the cylinder), that housed a 1 inch crystal. No consideration was given to hydrodynamics in the design of the transducer. Such flasher units were used for many years, and units were made that mounted in boat consoles, as well as more advanced portable units.
The next popular type of a transducer was one that was mounted on the transom of the boat with a metal bracket such that the transducer was a "permanent" part of the boat. The transducer had a flat bottom and a rounded or semi-circular nose (in plan view). The transducer would allow the fisherman to read the bottom at speeds to 15 or 20 mph. This transducer also used a 1 inch crystal. Because of the flat bottom with sharp edges, the transducer could not be used at very high speeds. The unit could be bonded inside the hull which provided increase in the operating speed range, dependent upon the mounting location and the flow disturbances underneath the hull.
The next transducer in the evolution being considered had a pointed nose and some effort was directed to providing hydrodynamic streamlining. However, this transducer was again flat-bottomed with a knife edge leading edge and the only way the transducer could be used at speed was with the transducer disposed at a 6 to 8 degree positive (relative to the oncoming water) angle of attack. Speeds up to 40 or 55 mph were possible in this orientation or configuration of the transducer. This unit was designed to be mounted on the transom of the boat, and could be mounted nose forward, or could be turned around "backwards" and mounted with the angled back against the transom of the boat. In this way, the bottom of the transducer could be made flush with the bottom of the boat and thus act as an extension of the boat hull. In this "backward" configuration, the transducer could operate at much higher speeds than with the nose forward. This transducer could also be mounted inside the hull in a "through-the-hull" configuration.
Although the discussion above concerns previous designs of the assignee, most competitive transducer units (including those, for example, made by Humminbird, Airmar Technology Corporation, and Radarsonics Inc.) are similar and, in particular, generally include flat bottoms and knife edges, although some provide minor radiusing or rounding of the leading edges. However, none of these competitive designs can operate at very high speeds unless mounted inside the boat hull or in some cases, mounted as an extension of the hull. Despite the obvious speed advantages of mounting any of the above transducers inside the hull, a major disadvantage is the signal attenuation and resultant loss of power which occurs in attempting to transmit through the thick fiberglass of the hull, because of discontinuities in the hull and because the transducers were not designed to accommodate the thick fiberglass in front of the crystal face.
The last design referred to above was used from the mid 1970s to the present, a roughly 15 year history of use, with little attempt at, or need for, improvement during that period of time. The main reasons for this are that since all transom mount transducers were limited in high speed performance (unless a great deal of experimentation was done to optimize the installation), this was considered to be the "nature of the beast," and there was no real pressure on manufacturers to provide improvement. As boat engine power increased and boat hull designs improved, the available speeds became substantially higher and the speed limitations of the early transducers became evident particularly with respect to transom-mount transducers. As discussed above, transom-mount transducers have the advantages that such transducers can be moved from one boat to another after a fisherman sells or trades his boat and that the transducers generally require less energy to excite them because the acoustical signal does not have to traverse through the thickness of the hull. Som "through-the-hull" transducers do not have the disadvantage associated with transmission through the hull because the transducer is actually molded into the hull on the outside of the hull envelope. These transducers are generally installed at the boat manufacturer or specialized marine shop and, depending on where the transducer is located and how "quiet" the hull is, these transducers can perform quite well at any speed of which the boat is capable. However, such transducers are of specialized application and obviously do not have the advantage of being able to be moved from boat to boat.
Hull "quietness" obviously has an important influence on the effectiveness of a transducer, and generally involves two phenomena, viz., hydrodynamic disturbances and structural excitation. The term hydrodynamic disturbances is used to refer to flow separation (cavitation, bubble generation, and the like) occurring before or at the transducer location that results in the transducer signal being absorbed, reflected, or refracted and, in any event, generally diminished in strength as compared with a transducer immersed in a homogeneous water environment.
The term structural excitation is used to refer to mechanical vibration of the boat hull which can excite the transducer crystal at or near the excitation frequency that the crystal is designed to "listen" for. As most fish-locating transducers are excited well above the range for human hearing (e.g., at 50 kHz, 100 kHz, 192 kHz, 200 kHz, 455 kHz, and so on), only structural excitations in that range will affect the transducer. In general, the higher the rigidity of the boat hull and the mounting assembly which mounts the transducer to the boat hull, the higher the natural frequency of the structural combination, and the greater the possibility of structural feedback. Aluminum hull boats are much more susceptible to such feedback because of the natural frequencies of the hull. The hull frequency is primarily excited by the engine and propeller combination but the hull is also vibrated by the collisions occurring between the hull bottom and waves or wavelets at speed. Transom-mount transducers therefore have an advantage over in-hull transducers in that the structural, i.e., mechanical, attachment to the hull can be designed to minimize the structural feedback path.
In choosing an excitation frequency, a transducer designer must choose between conflicting goals, viz., increased definition at higher frequencies versus increased depth capability at lower frequencies. The lower frequencies often employed for depth are, however, more susceptible to outside acoustical-structural interference. A good mounting system that isolates the crystal from structural influences is obviously important.
Years of experience with older transducer units has shown that, even as installed by a skilled installer, few of these transducers can operate reliably at speeds above 20 mph while being mounted s that the axis of the cylindrically shaped crystal is perpendicular to the surface of the body of water when the boat is traveling at trolling speeds (1 to 3 mph), a mounting orientation which is necessary to provide an undistorted sonar picture of what is directly below the boat (images of fish are characteristically uniform arches on the display screen). The transducers either have to be tuned for operation at trolling speeds (where the transducer has an angle of attack of about negative 2 to 5 degrees relative to the bottom of the hull so as to extend parallel to the water surface) or for operation at speeds above 20 mph (wherein the transducer has a positive angle of attack of up to 15 degrees). Older prior art transducers, because of such factors as the flat-bottomed shape thereof, the necessity to position the transducer below the boat hull, and the excessive positive angle of attack at high speed have been found to exhibit excessive fluid drag and to cause boat control problems (e.g., boat "lift") at higher speeds. Even with considerable adjustment or tinkering, many of the older transducers would not operate reliably above 30 mph.
In recent years, with the increased popularity of high-speed (40 mph and more) fishing boats, it has become desirable to locate fish, and the bottom, reliably at high speeds with a minimum of user installation expertise. Additionally, at these higher speeds, any additional hydrodynamic drag or lift forces (and especially drag or lift forces that are off-centerline) become particularly undesirable with respect to considerations such as power, top speed, and control.
A very serious disadvantage of older transducers is the common lack of hydrodynamic streamlining. Most of such transducers suffer from either a total absence of shaping for this purpose or from what can be worse, shaping that is based on "perceived hydrodynamics," i.e., shaping that is thought to provide a good hydrodynamic response but does not (e.g., transducers having an arrow shape in plan). The large drag and lift coefficients of these prior art devices detrimentally affect both the top speed of the boat and the effective control to be had over the boat during operation. As noted above, such transducers almost universally have a "flat bottom" shape and this leads to separated flows beneath the crystal and thus to severely attenuated signal strength (sometimes referred to as "loss of bottom"). A recent flow-visualization study has shown that such "loss of bottom" occurred at the exact time that separated flow was observed with an underwater video camera at speeds from 1 to 38 mph.
Another disadvantage of older prior art units concerns the mounting brackets for these units. In this regard, the mounting brackets for most older units do not allow for "kick-up" in the event a log or other underwater object is struck, i.e., do not provide for pivoting or other movement of the unit out of the way after being struck by such an object so as to prevent any damage to the unit. It will be appreciated that with a mounting not having such a "kick-up" feature, damage to the transducer unit resulting from such a collision is more likely. Further, most of the transducer mounting designs that do "kick up" are merely bolts with lock washers that permit the transducer to pivot or rotate out of harm's way. The user is then required to stop, perhaps trailer the boat, and properly reposition the transducer. However, the mounting assemblies of some relatively recent transducer models include a stop or abutment against which a portion of the transducer abuts in the lowered position so that the transducer can be returned to the lowered position in the event of "kick-up" occurring. Another disadvantage of some prior art mounting brackets is that the brackets are so stiff that the acoustic signals from the transducer pass harmonic vibrations from the boat hull to the receiving crystal, thereby adding noise to the sonar signal.
U.S. Pat. No. 4,907,208 (Lowrance et al) discloses a sonar transducer assembly which overcomes some of the disadvantages and deficiencies of prior units that were discussed above. The transducer body or housing is generally bullet-shaped and comprises a cylindrical main or base portion and a generally ellipsoidal nose, and includes three transducers inside the housing or body which are aimed in different directions below the boat. Although this transducer unit represents a significant improvement over the prior art with respect to the streamlining provided, the transducer device of the present invention possesses a number of advantages as compared with this unit, as is discussed below.